Toluenediamines are intermediates for the preparation of tolylene diisocyanates (TDI) which are important intermediates produced on an industrial scale for the preparation of polyurethanes. The preparation of these by catalytic hydrogenation of dinitrotoluenes is known and has been described many times (Ullmann's Enzyklopädie der technischen Chemie, 4th edition, volume 7, page 393 ff, 1973, Verlag Chemie Weinheim/New York). The industrial preparation of toluenediamines is carried out predominantly by reaction of a mixture of isomeric dinitrotoluenes (hereinafter also referred to as DNT) obtainable by nitration of toluene by means of nitric acid. Commercial mixtures of isomeric dinitrotoluenes are predominantly prepared as crude DNT using nitric acid in the presence of sulfuric acid as catalyst in a two-stage isothermal nitration process with intermediate formation of the corresponding mononitrotoluenes. They are subsequently worked up in steps following the reaction, predominantly in scrubs, and thus largely freed of dissolved sulfuric and nitric acid contents and also of secondary components formed in the reaction stages, e.g. cresols and degradation products thereof.
The catalytic hydrogenation of commercial DNT products can be carried out with concomitant use of an inert solvent or in bulk, with the mixtures then being melted before carrying out the hydrogenation. It can be carried out either batchwise or continuously using conventional reactors. The economic success of the process employed depends substantially on not only a continuous reaction but especially also the selectivities of the reaction which can be achieved in the process employed and also the capacities and operating lives of the catalysts used.
U.S. Pat. No. 3,356,728 discloses an improved continuous process for preparing aromatic amines by catalytic hydrogenation of aromatic polynitroaromatics in a slurry phase reactor, with the process being described by way of example for the reaction of dinitrotoluene. According to the teaching of U.S. Pat. No. 3,356,728, the catalytic hydrogenation of dinitrotoluene in this reaction system is particularly effective in terms of selectivity, catalyst operating life and throughput when                the reaction zone is always saturated with hydrogen during the reaction,        the aromatic polynitro compound is introduced into the system with maintenance of a particular weight ratio to the catalyst present in the reaction system (“catalyst loading”) and        the concentration of the aromatic polynitro compound introduced does not exceed a prescribed limit value in the reaction zone.        
According to the teaching of U.S. Pat. No. 3,356,728, the catalyst loadings claimed lead to high concentrations of active catalyst in the reaction system, so that the aromatic polynitro compound fed in is immediately converted into the desired amine after being introduced into the mixture, as a result of which the concentration of unreduced nitro compound in the reaction system is kept below 0.005% by weight at all times. According to the teaching of U.S. Pat. No. 3,356,728, rapid poisoning of the catalyst is prevented by this low concentration, and higher yields and an improved product purity combined with significantly reduced costs are also obtained in the reaction of the aromatic polynitro compound. The document does not disclose that the catalyst consumption can be reduced by the DNT to be hydrogenated being subjected to a particular treatment with application of an electric potential.
WO 02/062729 A2 is concerned with improving the mass transfer processes in heterogeneously catalyzed hydrogenation reactions, for example the preparation of aromatic amines from the corresponding nitro compounds. A higher hydrogen saturation of the liquid phase in the reactor is said to be achieved by means of a particular proportion of inert gas in the hydrogenation hydrogen and is said to prevent increased aging of the hydrogenation catalyst and unsatisfactory selectivity of the reaction. Finally, it is also an objective of this publication to ensure very complete conversion of the nitro compound into the amine in the hydrogenation reactor.